1. Field of the Invention
The present invention relates to a method and an apparatus for the process control which combines a feedback control and a feedforward control.
2. Description of the Prior Art
In the field of process control, demand has been increasing in recent years for an adaptive control method which is capable of adapting itself to the changes in disturbances, such as the variations in the load, environment, and the like, by responding to them in a flexible manner.
Such an adaptive control method requires as its fundamental conditions that it (1) can minimize the variations in the process variable even when the disturbance is a sudden change, (2) can respond optionally to any of the variations in a dynamic characteristic of the process, and (3) has a simple and multipurpose construction.
As representative examples of the prior art adaptive control method there are known (1) the self-tuning regulator (STR) and (2) a model reference adaptive control system (MRACS). However, such a control method corrects the parameters of the regulating system based on a mathematical analysis such as a fixing method or a stabilization theory, by focusing attention on the controlled result, namely, the changes in the process variable. That is, it is based on the feedback control which works on the availability of the controlled result, so that it is affected directly by the disturbances. Because of this, it is applicable only to those processes in which the disturbances, process characteristics, and the like vary slowly with special patterns, and cannot satisfy the above three fundamental conditions for the case of ordinary processes which vary in various manners.
In addition, in order to be able to respond flexibly to the variations in the load and environment, it is proposed in the prior art process control methods to combine the feedback control (called FB control hereafter) method which is the basis of the prior art process control method and the feedforward control (called FF control hereafter) method which estimates and controls beforehand the influence, such as the load variations, due to disurbances, to improve the ability to cope with the disturbance, namely, the response speed.
In the feedforward/feedback control (called FF/FB control hereafter) which combines the feedback control and the feedforward control, the FF control detects the disturbance such as the variations in the load, estimates and controls beforehand the process while compensating for the influence due to the disturbance by the disturbance compensation signal (this becomes the output of the FF control) from the FF control model, and corrects and controls the control deviation in the result by the FB control. Namely, in the FF/FB control, the FB control outputs an adjustment control (the FB control output) to correct the deviation when there is a control deviation as a result of the FF control, and hence, the FB control function will not be needed if the FF control is achieving an ideal operation for estimation and precursory control.
In an actual FF/FB control of a process, however, it is difficult to accomplishs an ideal FF control due to (1) the errors in the FF control model, (2) changes with the passage in time and changes in the enviroment, (3) influence of the disturbance elements not yet detected, and (4) influence of other indeterminate elements, and so forth, so that support from the FB control becomes necessary.
As it is, if the degree of contribution of the FB control to the FF/FB control is increased, there arise fatal weaknesses for its practical use in the process control, such as (1) a decrease in the ability to cope with, and the controllability of, the disturbance, and (2) a marked decrease in the responsiveness to the changes in the disturbance due to incomplete FF control, and so on, for its practical use in the process control.